The present invention generally relates to an apparatus and method for carrying out catalytic reactions at elevated temperatures and more particularly to catalytic reactions in a steam-hydrocarbon reforming process.
A steam-hydrocarbon reformer is a furnace utilized to generate sufficient heat to drive an-endothermic reaction in fluid that is passed through catalyst filled reaction tubes disposed within the furnace enclosure. Fluid, in the form of a mixture of steam and hydrocarbon, flows to the reaction tubes located internally of the enclosure. Heat is produced in the enclosure by burning fuel to produce hot combustion gas. Heat is transferred to the fluid flowing through the reaction tubes indirectly via radiation and convection at a level sufficient to produce a hydrogen rich effluent gas consisting essentially of hydrogen and carbon monoxide.
The mixture of steam and hydrocarbon is introduced to the reaction tubes according to a fixed steam to hydrocarbon-carbon ratio at a temperature between about 700xc2x0 F. and 1,000xc2x0 F. In the reaction tubes, the mixture of steam and hydrocarbon is quickly heated to the reaction temperature and converted to a hydrogen rich effluent gas in the presence of a reforming catalyst. The effluent gas is generally withdrawn from the reaction tubes at a temperature approximately between 1,400xc2x0 F. and 1,550xc2x0 F.
In most prior art arrangements, multiple once-through catalyst filled reaction tubes are utilized for steam-hydrocarbon reforming. In once-through reaction tubes, the mixture of steam and hydrocarbon enters at one end of each reaction tube from a distribution header located externally of the furnace enclosure. The distribution header is connected to each reaction tube via semi-flexible conduits. The fluid flows through the reaction tubes, in contact with the reforming catalyst, and exits at the opposite ends through semi-flexible conduits connected to an outlet collection header, also located externally of the furnace enclosure.
The required reaction tube heat transfer surface area is determined from the specified average radiant heat flux. The specified average heat flux is a function of the type of burner firing arrangement employed. For example, a catalyst tube fired on only one side may utilize an average heat flux of approximately between 13,000 Btu/hr-ft2 and 17,000 Btu/hr-ft2 to limit the thermal stresses associated with uneven heating around the reaction tube circumference. However, a reaction tube fired on two or more sides may utilize a higher average heat flux of approximately between 23,000 Btu/hr-ft2 and 35,000 Btu/hr-ft2.
Once-through reaction tubes have been employed in both box and cylindrical reforming furnace enclosures generally suitable for medium to high production capacity. In box reforming furnaces, once-through reaction tubes are typically arranged vertically in either a single straight row along the center of the enclosure or in multiple straight rows evenly spaced within the enclosure. Burners used to supply flame and hot combustion gas may be arranged along the walls, floor or roof of the enclosure.
In cylindrical reforming furnaces, once through reaction tubes a re arranged vertically in either a single straight row along the center of the enclosure, in a cross pattern along the enclosure centerline, in multiple rows evenly spaced within the enclosure, or in a circular pattern along the walls of the enclosure. Either a single bumer or multiple burners may be arranged i n the floor or roof of the enclosure.
One common problem with prior art steam-hydrocarbon reformers is that a high uniform heat flux is required to quickly raise the fluid temperature to its reaction temperature and maintain it at approximately between 1,400xc2x0 F. and 1,550xc2x0 F. As a result, an uneven tube wall temperature profile develops along the length of each reaction tube due to the flow patterns of the combustion gas inside the furnace enclosure and the process fluid inside the reaction tubes. At the reaction tube outlets, where the process fluid has little remaining capacity to absorb heat, excessively high tube wall temperatures cause carbon deposits to form on the surfaces of the catalyst and on the inside surfaces of the reaction tubes. This results in reduced catalyst activity and damage to the reaction tubes.
U.S. Pat. No. 4,324,649 discloses a steam-hydrocarbon reformer that controls reaction tube wall temperature profiles by allowing the combustion gas to flow in a back mixed flow pat tern in the bottom portion of the furnace enclosure and a plug flow pattern in the top portion of the enclosure. This two-zone approach is limited to use in natural draft furnaces where pressure drop caused by channeling the flow of combustion gas results in turbulent upstream flow patterns. Unfortunately, in natural draft furnace arrangements, convection heat exchange equipment must be mounted above the furnace combustion gas outlet to recover the waste heat in the combustion gas exiting the furnace enclosure. This requires the construction of a large structure to support the furnace enclosure, the convection heat exchange equipment and the combustion gas stack.
Another problem with prior art arrangements is that average radiant heat flux, not reaction kinetics, is the controlling factor in determining the number and size of the reaction tubes. Achieving the required reaction tube heat transfer surface area for the specified average heat flux results in an excessive volume of catalyst required to affect the reaction, lower process fluid velocities inside the reaction tubes, and increased process fluid residence time inside the catalyst bed. These conditions contribute to inefficient heat transfer within the reaction tube and uneven heating of the reaction tube walls.
Yet another problem with prior art arrangements is the requirement for high alloy semi-flexible connecting conduits at the reaction tube outlets. Stresses caused by thermal expansion must be relieved by providing pigtail, trombone loop or omega loop connections between the reaction tube outlets and the collection header. Additional piping flexibility must be provided between the collection header and the downstream process equipment.
The most serious problem with the prior art steam-hydrocarbon reformers is that the cost of unutilized catalyst and extra reaction tubes, the cost of a complex high alloy reaction tube outlet piping system, and the cost of building vertically to obtain the benefits of natural draft cannot presently be justified for low production capacity steam-hydrocarbon reformers. Thus, a need still exists for a cost effective steam-hydrocarbon reformer suitable for use in the low production capacity environment such as where the maximum desired production capacity of hydrogen rich effluent gas is approximately 24,000 standard cubic feet per hour on a dry gas basis.
The steam-hydrocarbon reformer of the present invention includes: (a) a furnace enclosure with at least one burner located inside the enclosure; (b) a vertical reaction tube located internally of the enclosure with a center return tube nested within the reaction tube and a reforming catalyst packed between the reaction tube and the center return tube; and (c) a process fluid cooler located externally of the enclosure with the center return tube outlet directly connected to the process fluid cooler inlet.
In another aspect of the invention, the above described steam-hydrocarbon reformer is used in a process for producing a hydrogen rich effluent gas by: (a) heating the furnace enclosure by burning a flammable gas or liquid inside the enclosure with the burner(s) so as to produce a combustion gas; (b) flowing the combustion gas through the furnace enclosure under an induced draft; (c) introducing a mixture of steam and hydrocarbon into the reaction tube; (d) converting at least a portion of the steam and hydrocarbon mixture to a hydrogen rich effluent gas; (e) flowing the hydrogen rich effluent gas through the center return tube; and (f) cooling the hydrogen rich effluent gas in the process fluid cooler. In a preferred embodiment, the above described steam-hydrocarbon reformer produces a maximum of approximately 24,000 standard cubic feet per hour of hydrogen rich effluent gas on a dry gas basis.
Advantages of the invention will be obvious from the description, or may be learned by practice of the invention. Additional advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.